Commercial dry cleaning systems currently employ potentially toxic and environmentally harmful halocarbon solvents, such as perchloroethylene. Carbon dioxide has been proposed as an alternative to such systems in U.S. Pat. No. 4,012,194 to Maffei. A problem with carbon dioxide is, however, its lower solvent power relative to ordinary solvents.
Gennan Patent Application DE3904514 A1, published Aug. 23, 1990, describes a cleaning system combining various conventional anionic or nonionic surface active agents with supercritical CO.sub.2. The system described therein appears to combine the detergency mechanism of conventional surface active agents with the solvent power of supercritical fluid carbon dioxide. A carbon dioxide dry cleaning system effective for liquid carbon dioxide is not provided.
U.S. Pat. No. 5,683,473 to Jureller et al. (see also U.S. Pat. No. 5,683,977 to Jureller et al.) describes a dry cleaning system utilizing carbon dioxide in liquid form in combination with surfactants that contain a functional moiety that is CO.sub.2 -philic, which surfactants are not conventionally used for detergent cleaning. Since there are numerous advantages to employing conventional surfactants (e.g., cost, ready availability, established regulatory approval, established toxicology, etc), it would be extremely desireable to have a dry cleaning system for liquid carbon dioxide that employs conventional surfactants that do not contain a CO.sub.2 -philic group.
U.S. Pat. No. 5,377,705 to Smith et al. describes a precision cleaning system in which a work piece is cleaned with a mixture of CO.sub.2 and a co-solvent. Smith provides an entirely non-aqueous system, stating: "The system is also designed to replace aqueous or semi-aqueous based cleaning processes to eliminate the problems of moisture damage to parts and water disposal" (col. 4 line 68 to col. 5 line 3). Co-solvents that are listed include acetone and ISOPAR.TM. M (col. 8, lines 19-24). Use in dry cleaning is neither suggested nor disclosed.
In view of the foregoing, there is a continuing need for effective carbon dioxide-based dry cleaning systems.